1. Field of the Invention
The invention relates to a method for controlling corrosion in boiler systems and, more particularly, to treating boiler water to remove dissolved oxygen and to passivate metal surfaces, while providing a composition which is stable at room temperature over six months.
2. Description of the Prior Art
Efficient operation of boilers and other steam-run equipment requires chemical treatment of feedwater to control corrosion. corrosion in such systems generally arises as a result of oxygen attack of steel in water supply equipment, pre-boiler systems, boilers, and condensate return lines. Oxygen attack of steel is exacerbated by the unavoidable high temperatures found in boiler equipment. Since acidic conditions also accelerate corrosion, most boiler systems are run in an alkaline environment.
The action of dissolved gases such as oxygen and carbon dioxide are two of the main factors that lead to feedwater system and boiler corrosion. In order to understand the role of dissolved gases in corrosion, one must understand the electrochemical nature of corrosion.
Corrosion processes involve reactions where one species is oxidized EQU M.fwdarw.M.sup.2t +2e.sup.-
and another is reduced. EQU x+e.sup.- x.sup.-
In boiler systems the two species involved in the redox chemistry are typically two different metals, a metal and oxygen, or a metal and water. Under most conditions, oxidation of iron occurs. EQU Fe.sup.0 Fe.sup.2t +2e.sup.-
A current of electrons then flows from this anodic region to a point where reduction takes place. If oxygen is present, the cathodic reaction is EQU O.sub.2 +H.sub.2 O+4e.sup.- 4OH.sup.-
In the absence of oxygen, water is reduced to hydrogen. EQU 2H.sub.2 O+2e.sup.- H.sub.2 +2OH.sup.-
Any agent that inhibits either the anodic or cathodic reaction will stop corrosion from occurring. Metal passivation, the formation of a protective oxide film, is one common example of a process that inhibits corrosion by blocking one of the electrochemical reaction pathways.
The severity of oxygen corrosion will depend on the concentration of dissolved oxygen in the water, water pH and temperature. As water temperature increases, corrosion in feed lines, heaters, boilers, steam and return lines made of iron and steel increases.
In most modern boiler systems, dissolved oxygen is handled by first mechanically removing most of the dissolved oxygen and then chemically scavenging the remainder. Mechanical degasification is typically carried out with vacuum degasifiers which will reduce oxygen levels to less than 0.5-1.0 mg/L or with deaerating heaters, which will reduce oxygen concentration to the range of 0.005-0.010 mg/L.
Chemical scavenging of the remaining dissolved oxygen is widely accomplished by treating the water with hydrazine. See, for example, the Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, Interscience Publishers, Volume II, page 187. As explained in Kirk-Othmer, hydrazine efficiently eliminates the residual oxygen by reacting with the oxygen to give water and gaseous nitrogen. In addition, hydrazine is a good metal passivator since it forms and maintains an adherent protective layer of magnetite over iron surfaces.
It is, however, widely recognized that hydrazine is an extremely toxic chemical. Kirk-Othmer reports that it is highly toxic and readily absorbed through the mouth, skin and respiratory system, and that permanent corneal damage may result from contact with the eye. Low doses may cause central nervous system depression and high doses may cause, convulsions and other damaging side effects.
Thus, it is an object of this invention to provide oxygen scavenging treatments which are free of or reduced in the dangers inherent to hydrazine, but which scavenge oxygen and passivate steel surfaces under typical boiler use conditions.